כִּ֞י אֶת־מַעְשַׂ֣ר בְּנֵֽי־יִשְׂרָאֵ֗ל אֲשֶׁ֨ר יָרִ֤ימוּ לַֽיהוָה֙ תְּרוּמָ֔ה נָתַ֥תִּי לַלְוִיִּ֖ם לְנַחֲלָ֑ה עַל־כֵּן֙ אָמַ֣רְתִּי לָהֶ֔ם בְּתוֹךְ֙ בְּנֵ֣י יִשְׂרָאֵ֔ל לֹ֥א יִנְחֲל֖וּ נַחֲלָֽה׃ (פ)
Sapagkat ang ikapu ng mga anak ni Israel, na kanilang ibinubukod bilang handog para sa Walang-Hanggan, ay ibinigay Ko sa mga Levita bilang mana; kaya nga sinabi Ko sa kanila: “Sa gitna ng mga anak ni Israel ay hindi sila magkakaroon ng mana.”
Rashi
אשר ירימו לה׳ תרומה [BUT ALL THE TITHES OF THE CHILDREN OF ISRAEL] WHICH THEY HEAVE AS A HEAVE OFFERING UNTO THE LORD — Scripture terms it (the tithe) T’rumah (although it is not holy and may be eaten by a layman) until he (the Levite) sets apart from it the “heave offering of the tithe” (Sifrei Bamidbar 119:5).
Or HaChaim
כי את מעשר בני ישראל אשר ירימו, for the tithes of the children of Israel which they set aside, etc. Earlier the Torah had said: "for I have given all the tithes to the Levites, etc. (18,21)." How do we reconcile this repetition of the same legislation? It appears that there are two aspects to the tithes which G'd assigned to the Levites. One reason they receive the tithes is in compensation for the Levites not having received a share of the land. The second reason is to save them having to till the land, i.e. the seven stages of work until the farmer's wheat is finally ready to be milled, etc. This is the reason the Torah gives a different reason in verse 21 i.e. that it is given to them in exchange for the service they have to perform in the Tent of Testimony.
Siftei Chakhamim
Scripture calls it “a terumah-gift” until he separates. For if not so, why would the verse have to say “terumah-gift (תרומה)” surely it had already said, “Which they must set aside (ירימו) to Hashem,” implying that one must set aside a portion from it. Furthermore, at the beginning the verse calls it a “tithe” while at the end it is called “terumah.”